Various embodiments of methods for chemical vapor infiltration (CVI) are known. Procedurally the simplest to perform are methods of isobaric and isothermic chemical vapor infiltration. In this method the entice process space exists at constant temperature and pressure. Here, however, only very low pressures or partial pressures of educt gases can be used, when necessary with addition of inert or dilution gases, so that extremely long infiltration times are required.
In order to shorten the infiltration times, chemical vapor infiltration of silicon carbide uses methyltrichlorosilane (MTS) as educt gas, the educt gas being preheated to temperatures well above the decomposition temperature of MTS i.e. above about 100° C. while at the same time setting pressures up to about 25 kPa and removing silicate components from the gas phase at the outlet of the reaction zone. Preheating the MTS to such high temperatures leads to a high rate of deposition of the substances added with the gas. This achieves a high production speed but at the same time leads to uneven deposits, particularly on the surface, and to minimal pore filling. Thus optimal or maximal pore filling is more effective at extremely slow deposition or infiltration rates (e.g. W. V. Kotlensky, in Chemistry and Physics of Carbon, Vol. 9, edited by P. L. Walker and P. A. Thrower, Marcel Dekker, New York, 1973, pg. 173).
In order to successfully realize infiltration, low pressures, and particularly low partial pressures, are recommended. The pressures under the conditions of industrially applied chemical vapor infiltration are at least one to two orders of magnitude below normal pressure. Starting compounds are partially mixed with inert gases so that their partial pressure, and the deposition rate, can be further lowered. Due to the low partial pressures, extremely long infiltration times of up to several weeks are required.
Since the isobaric and isothermic methods failed in achieving rapid production and high degrees of pore filling, the development of new methods have been attempted, such as pressure gradient, temperature gradient and pressure switching methods. Such methods are for example known from Nyan-Hwa Tai and Tsu-Wei Chou, Journal of American Ceramic Society 73, 1489 (1990).
In the vacuum pressure pulsation method, the process pressure is continually varied to support the diffusion. The disadvantage of this method lies in the cost of the apparatus as well as in the filtration times which are still very long.
Another well-known method is the temperature gradient method for example, as disclosed in U.S. Pat. Nos. 5,411,763 (Weaver et al.) and 5,348,774 (Golecki et al.). In this method, heat is removed from the side of the porous substrate facing the process gas stream by suitable measures, for example by cooling by the stream. The side of the porous substrate opposite to the gas stream is adjacent to a heating element. It is in this way that a temperature gradient crucial to the method is established normal to the surface of the substrate. The surface temperature on the cold side is adjusted with the gas stream such that very little or no deposition takes place. It is in this way that narrowing of the pores in this region is avoided. The disadvantage of this method is the very high gas throughput necessary for cooling. The low yield of deposited material entails long production times. Much equipment is needed for the heating.
In a further known embodiment of CVI methods, the gas is streamed through the porous substrate with forced convection whereby a pressure gradient is established. The infiltration time can be kept relatively short. However, after a certain level of pore filling, the streaming through of the porous structure becomes more difficult.
U.S. Pat. No. 4,580,524 (Lackey et al.) discloses a CVI method whereby temperature and pressure gradient techniques are combined with one another. In this way relatively short production times can be achieved. The disadvantage of such a method is the complicated reactor construction.
The task which provided the basis for this invention is to create a CVI method by which a high level of pore filling during a pre-set production time can be achieved, or alternatively, a shorter production time achieved for pre-set pore filling levels.
Silicon carbide is useful for its very good mechanical, physical, and chemical properties including corrosion resistance, chemical stability, and temperature stability. The present invention provides a method of producing an article of manufacture that comprises silicon carbide shells. The method comprises the following major processes, 1) manufacture silicon carbide shells around polystyrene sacrificial cores, 2) providing a means of agitating the silicon carbide shells to prevent them from inadvertently bonding to one another during the chemical vapor deposition and core burn-out processes, 3) chemical vapor deposition (CVD) to deposit silicon carbide with flowing methyltriclorosilane, (CH3SiCl3) MTS in hydrogen over a substrate heated to about 100° C. to about 1400° C.
Methods for producing composite materials are disclosed in the prior art. U.S. Pat. No. 6,197,374 (Huttinger et al.) describes an isobaric and isothermic method of chemical vapor infiltration of refractory materials into a porous substrate in a reaction zone. The method comprises deposition on the porous substrate within the reaction zone, providing a linear flow of an educt gas comprising deposable material in the reaction zone at a reaction temperature and a reaction pressure that produce saturation adsorption of the deposable material onto the substrate the linear flow being adjusted to have a flow rate such that no more than 50% of the deposable material is deposited into the porous substrate. Methods for producing composite materials are disclosed in U.S. Pat. No. 6,197,374 (Huttinger et al.) incorporated herein by reference.